When frog sartorius muscles are exposed to methadone (2-4 X 10(-4) M) the action potential recorded intracellularly is depressed and eliminated in 3 to 4 hr. This is due to a decrease in the sodium conductance as measured by the maximum rate of rise of the action potential. When the calcium concentration in frog Ringer's solution (normally 1.08 mM) was either lowered to 0.54 mM or raised to 2 mM, the effect of methadone on excitability was unchanged. However, increasing the extracellular calcium to 4 mM decreased the action of methadone. As this would also increase the intracellular calcium concentration, in other experiments the free intracellular Ca+ concentration was raised by adding 0.4 mM caffeine to the Ringer's solution. This also antagonized the depressant action of methadone. The antagonistic action of caffeine was not due to a direct effect of caffeine on sodium conductance because the administration of caffeine by itself caused a 10% depression of the action potential maximum rate of rise and caffeine did not antagonize the action of tetrodotoxin, which is a specific sodium channel blocking agent. Finally, it was shown that the calcium ionophore, A23187, in low concentrations antagonized the depressant actions of methadone and meperidine on action potential production. It was concluded that increasing the intracellular free calcium concentration antagonized the depressant effect on excitability produced by opiate drugs acting on an intracellularly oriented opiate drug receptor.